1. Field of the Invention
The present invention relates to a radio paging system with voice transfer function and a radio pager for use therewith. More particularly, the invention relates to a unidirectional voice message transmission system comprising a paging station and a receive-only radio pager, the system allowing the radio pager carried by a subscriber to receive not only paging calls but also voice messages coming from the paging station.
2. Description of the Prior Art
Conventional radio paging systems transmit paging calls to a subscriber carrying a radio pager by causing the pager to emit sound, intermittent light or vibration in response to signals sent via telephone and radio networks. Recently developed paging systems send to the radio pager not only paging calls but also predetermined messages containing numbers, symbols and kanji characters for display on the pager's screen. These systems are used not only on public telephone networks but also on private telephone networks installed in hospitals, hotels, department stores and senior citizens' homes.
Japanese Patent Laid-Open No. HEI/3-60227 discloses a voice message transmission system which, when a voice message signal is input via a telephone network, causes a paging station to convert the signal from analog to digital format. After conversion, the digital signal is stored and then forwarded via a radio network to a destination radio pager. In turn, the addressed radio pager stores the received digital signal in its message storage and then converts the signal back to analog format for voice playback.
In connection with the above voice message transmission system, no disclosure is made as to how to transmit large amounts of data within the technical and regulatory constraints of the current networks. For example, if a voice message is sampled in 8 bits at 8 kHz, the amount of data is as high as 64 kilobits per second. Of today's typical paging systems, those pursuant to the NTT specifications involve a channel separation of 12.5 kHz and a maximum frequency deviation of .+-.2.5 kHz; the systems complying with the POCSAG or Golay specifications require double the bandwidth of the NTT-compatible systems. It follows that transmitting 64 kilobits/sec. data illustratively at the transfer rate (400/sec.) of the conventional NTT-compatible transmission lines would take about 54 seconds. The process is too time-consuming for necessary voice information to be transmitted in practice.
In today's highly information-oriented society, there will be increasing occasions on which it is desired to keep transmitted messages from getting tapped by strangers. In such cases, a privacy function for tapping prevention is mandatory. At present, prior art voice message transmission systems have no such function.
Prior art systems have further disadvantages. On a typical radio pager capable of storing both character and voice information, the subscriber carrying the pager generally performs necessary operations thereon to get the character and/or voice information reproduced for display on the pager's screen or for voice playback. In that case, whether the transmitted message is a mixture of both character and voice information or contains either only, the addressee has no means of knowing the whole stored message until the entire series of operations is carried out to read the number from the display or hear the reproduced voice information from the pager's speaker. Although the presence of character information is seen simply at a glance on the pager display, the presence of a voice message is known only if a playback operation is performed. During voice playback, the message may be so long that the addressee may become irritated, not knowing when it will end.
In addition, the radio pager is required to consume as low power as possible so that the battery incorporated therein will be replaced at the longest possible intervals. This is not always the case with prior art systems.